Introduction

The occurrence of obesity is determined by genetic, metabolic, and behavioural factors. The results of several epidemiological studies and meta-analyses, conducted in the last decade, have allowed us to identify a new risk factor of overweight and obesity, which is short duration of sleep [1–6]. In the opinion of the majority of authors, the lowest occurrence of excessive body mass in adults is related to sleep lasting at least 7 h [7–9]. However, so far, the association between longer sleep duration, the state of health, and the risk of obesity has not been fully explained [10–14]. Moreover, in studies conducted to date, a higher body mass index (BMI) has been adopted as an indicator of obesity. However, there have been few papers published in which the association between body composition and sleep duration was analysed, and their results are inconclusive [7, 15]. Likewise, the influence of gender on the association between sleep duration and the risk of obesity has not been fully explained [7, 16–18].

Aim of the research

The aim of the study is to examine relationships between self-reported sleep duration, BMI, and body fat percentage, and also to determine whether such associations are the same in men and in women.

Material and methods

Research material was collected within the framework of the PONS project (Polish-Norwegian Study), prospective research on the health condition of the inhabitants of the Świętokrzyskie Province in Poland. The study was approved by the Ethics Committee within the Cancer Centre and Institute of Oncology in Warsaw, and by the Committee on Bioethics at the Faculty of Health Sciences, Jan Kochanowski University in Kielce, Poland. The studies included a questionnaire interview, anthropometric measurements, blood pressure measurements, and analyses of collected fasting-blood samples, on the basis of which the concentration of cholesterol and triglycerides was determined. Detailed information regarding the project, research procedures, and group selection were described in previously published papers [19–21]. In brief: 13,172 individuals were examined (4447 men), aged between 37 and 66 years, permanently residing in the Kielecki Region in Poland. Individuals with incomplete data were excluded from the study (n = 2609), as well as people with a history of cardiovascular disease (coronary artery disease, angina pectoris, myocardial infarction), stroke, cancer, or diabetes (n = 196). In further analysis of the data, 10,367 participants were included.

Sleep duration

Sleep duration was assessed with the question: “On average, how many hours do you sleep each night?” Answers were recorded in whole hours. We created the following three categories of sleeping duration: ≤ 6 h, 7–8 h, and ≥ 9 h per night. We also refer to these three groups as short, normal, and long sleepers, respectively.

Anthropometric measurements

Anthropometric indices were measured by well-trained investigators, following a standard protocol. The measurements of body weight and percentage of body fat were done by means of the body composition analyser, Tanita SC 240 MA, with an accuracy of 0.1 kg and 0.1%. Body height measurements were done by means of the scales’ stadiometer, with an accuracy of 0.1 cm. Body mass index was calculated as weight in kg divided by the square of height in metres. Obesity was defined as a body mass index ≥ 30 kg/m2 and on the basis of sex-specific cutoffs for %BF at the level ≥ 25% for men and ≥ 35% for women [22].

Covariates

Covariates for model adjustment were selected according to known predictors of obesity and factors that have an influence on sleep duration [23, 24]. The socio-demographic variables included: sex (men; women), age, education (university; lower than university), place of residence (city; country), and marital status (married or in a stable relationship; single or a widow/widower). Physical activity (PA) was evaluated with the use of the International Physical Activity Questionnaire (IPAQ) – the long form [25]. The analysis included the most frequent forms of activity, i.e. walking PA and moderate PA. Due to the small number of participants declaring vigorous PA, we did not include it in our analysis. Walking PA during the last week involved walking for 10 min or more every day, in all domains subject to assessment: job-related PA, transportation PA, recreation, sport, and leisure-time PA. Moderate PA included the time devoted to activities of moderate intensity, related to the domains: job-related PA; housework, house maintenance and caring for family; sport, recreational and leisure time. The scores are presented as time in minutes/day. Sitting time (ST) during the preceding week was determined on the basis of time spent in a sitting position on working days and at weekends. Next, the average number of minutes spent sitting during the day was calculated. The data concerning coffee and alcohol consumption were collected by means of the Food Frequency Questionnaire (FFQ). Alcohol consumption was evaluated on the basis of the frequency of alcoholic drinks consumption during the preceding 30 days in the following categories: every day, 4–5 times a week, 2–3 times a week, once a week, 2–3 times in the last 30 days, once during the last 30 days, not at all in the last 30 days, I don’t know, refusal to answer. The answers relating to the consumption frequency of products from the questionnaire were transformed into daily consumption doses and then standardised by z-score. As far as coffee is concerned, a portion consisted of one cup (250 ml). The frequencies of consumption were classified as follows: 6 times a day or more, 4–5 times a day, 2–3 times a day, once a day, 5–6 times a week, 2–4 times a week, 1–3 times a month, once a week, less frequently than once a month or not at all, I don’t know, I refuse to answer the question. The respondents who smoked cigarettes on a daily basis during the study were classified as current smokers, and those who had not smoked for longer than 6 months – as former smokers; the rest were regarded as non-smokers.

Statistical analysis

The normality of distribution of quantitative characteristics was evaluated: %BF, BMI, age, coffee and alcohol consumption, PA, and ST. Arithmetic means and standard deviations as well as medians and a quartile range were calculated (Q1–Q3) in the groups distinguished, based on the time devoted to sleep. A structure indicator was calculated for qualitative characteristics: place of residence, education, marital status, and smoking. The 2 test was used to test the structure indicator, whereas in case of quantitative variables, a one-way analysis of variance ANOVA or the median test and Kruskal-Wallis test were applied, depending on the type of distribution and the significance of variance (Table 1). The Scheffé post-hoc test was used to evaluate intergroup differences. Logistic regression was used for risk assessment (OR) of the occurrence of abnormal values of BMI and %BF in individual groups of sleep duration. Sleep of 7–8 h per night was adopted as a reference level. Two models were analysed: unadjusted and adjusted for socio-demographic variables (age, gender, place of residence, education, marital status); and health-related behaviour (smoking, coffee and alcohol consumption, sum of moderate and walking PA, sitting time). The p-values less than 0.05 were considered statistically significant. The statistical analysis was carried out with the use of the Statistica software, version 12.0.

Results

The characteristics of the study participants is presented in Table 1. Female participants of the study sleeping the longest had a higher BMI and more adipose tissue compared to other women. In men, no significant difference of %BF was observed depending on sleep duration, whereas the highest BMI was noted in men sleeping the shortest. The lowest average age of women was found in those sleeping 7–8 h, and the highest average age was noted in the group of men sleeping the longest. Shorter sleep duration was significantly more often found in men and women with better education, living in cities, compared to inhabitants of rural areas. The group of those with a short sleep duration included the greatest number of single women compared to those in stable relationships. No differences were noted in relation to the amount of coffee and alcohol consumption, and in the number of smokers, depending on sleep duration. Women who slept 7–8 h were most physically active, while the shortest sitting time was noted in those who slept less than 7 h per night. The risk of obesity, determined on the basis of the BMI indicator, was higher in the general subject population, both in the group of subjects sleeping the longest, as well as those sleeping the shortest, compared to individuals sleeping 7–8 h (Table 2). In men, the risk of obesity was significantly higher only in the group sleeping ≤6 h, whereas in women this was true only in those sleeping ≥ 9 h. The same scores were obtained in the models unadjusted and adjusted for all confounders. The risk of obesity determined on the basis of %BF was higher only in individuals sleeping ≥ 9 h. In the adjusted model, it turned out to be significant in the general study population and in women.

Discussion

The results of the conducted study confirmed that the risk of obesity (BMI ≥ 30 kg/m2) was lowest in individuals declaring 7–8 h of sleep, i.e. the duration considered by most authors to be optimal for maintaining normal body mass [7, 26–28]. Chaput et al. [29] showed that a change of sleep duration from short (≤ 6 h per day) to one lasting 7–8 h was, in the period of a six-year-long observation, related to a decrease of adipose tissue gain. It was proven that shorter sleep can affect energy balance by an up-regulation of orexigenic hormones and a downregulation of anorexigenic hormones associated with increased hunger and caloric intake [30, 31]. Doo and Kim [32], in a study of the Korean population, confirmed that the association of short sleep duration with the risk of obesity was potentially changed by dietary fat and carbohydrate consumption. Lakerveld et al. [33] proved that shorter sleep duration can also lead to obesity due to the longer screen time related to it. The results of our own study revealed that men and women sleeping 7–8 h spent significantly more time doing moderate physical activity compared to other participants, which could be an additional factor decreasing the risk of excess body mass in this group of subjects.
Some papers did not confirm that sleep duration was significantly correlated with a greater risk of obesity [34, 35]. Grandner et al. [36] explain that the association between sleep duration and BMI is probably age-dependent. In young adults, this relationship is linear, and the longer the sleep, the lower the BMI. This association changes in middle-age, in which 7–8 h of sleep is connected with the lowest BMI, and both shorter and longer sleep are related to a higher BMI. In older individuals, correlations between these variables become weaker. Our subject age group (37–66 years) included mainly middle-aged participants and confirmed a U-shaped association between sleep duration and BMI characteristic for this time of life.
The results of the analyses of relationships between body composition and sleep duration are unambiguous. Chaput et al. [7] found lower percentage of body fat in men and women who reported sleeping 7–8 h/night compared to those reporting 5–6 h of sleep per night. However, the authors calculated body fat percentage based on the measurements of skinfold thicknesses. St-Onge et al. [15] did not find any significant correlation between adiposity and sleep duration in adults. However, Wirth et al. [37] noted that both elevated BMI as well as body fat percentage were observed for shorter sleep duration, whereas in both papers body fat percentage was measured by dual X-ray absorptiometry. Xiao et al. found that a short sleep duration (< 6 h) was associated with higher measures of body size and fat composition, although the effects were attenuated after snoring was adjusted [38].
The lack of relationship between short sleep duration and %BF, noted in this study, may result from the fact that a significant percentage of participants with BMI < 30 kg/m2 had excessive adipose tissue, which was confirmed by previously conducted analyses [39] and studies by other authors [40]. Correlations obtained in the analysis of each criterion of obesity are different because sex-specific cutoffs for %BF at a level ≥ 25% for men and ≥ 35% for women were, in the subject population, more comparable with the value of BMI ≥ 25 kg/m2 than ≥ 30 kg/m2.
The associations between long sleep duration and the risk of excessive body mass are relatively seldom described in literature. However, several authors confirmed that longer sleep duration is related to a greater risk of overweight and/or obesity and a greater mass gain [17, 41, 42]. Tu et al. [43] noted that participants who had higher measurements for BMI and waist circumference were less likely to have short sleep duration and more likely to have long sleep duration. However, Nagai et al. [44], in long-term studies, found that longer sleep was connected with long-term weight gain, but only in the obese. A reliable explanation may involve a reduced energy expenditure related to longer time spent in bed [18, 41, 42]. Moreover, longer sleep can result in the subjects having less time for physical activity in the day. We need to consider the possibility that self-reported long-duration sleepers are spending a lot of time in bed but not getting a lot of sleep, i.e. they might have poor sleep quality due to sleep disorders or other health issues [41]. Xiao et al. [38] observed that poor sleep quality was associated with higher adiposity. The association between long sleep duration and the risk of obesity was not confirmed by Kobayashi et al. [4], which could result from the fact that ≥ 8 h was adopted as cut-off of long sleep duration.
The analysis of scores conducted separately for both sexes revealed that in women a greater risk of obesity was related to longer sleep duration, whereas in men, shorter sleep duration. Moreover, this relationship in men occurred only when BMI was adopted as a criterion of obesity. The results obtained by other authors are inconclusive. Most papers showed that associations between sleep duration and BMI are stronger, or occur only in women, not in men [16, 17, 42, 45, 46]. St-Onge et al. [15] also state that the relationship between self-reported sleep duration and body composition may be stronger in women than in men. Other studies did not reveal any differences based on sex [4, 18]. However, a few papers showed that such associations were present only among men [7, 47]. Spaeth et al. [48] found that men exhibited a greater increase in daily caloric intake during sleep restriction as a result of consuming more calories during late-night hours than women did. Population studies showed that sleep duration was inversely associated with BMI in men only, whereas poor sleep quality was positively associated with BMI in women only [49, 50].
Due to the cross-sectional nature of the study, a causal connection between sleep duration and the risk of obesity cannot be fully explained. However, experimental studies have shown that sleep reduction may have a significant metabolic effect on body mass homeostasis [51]. A limitation of this study involves also the fact that sleep duration was not measured but obtained from a questionnaire. However, Taheri et al. [52] found that self-reported sleep duration and polysomnographic measurement are both stable and highly correlated. Moreover, the study did not include the quality of sleep, which, similar to too short sleep, can activate the same mechanisms of energy uptake regulation and cause greater adiposity [53, 54].
A strong aspect of the study involves the large number of subjects and the fact that it was a homogeneous group in relation to age and ethnic origin. The analysis also included a large number of confounders, such as physical activity, sitting time, coffee consumption, and socio-demographic variables.

Conclusions

The conducted study confirmed the existence in the general study population of a U-shaped relationship between sleep duration and the risk of obesity defined as BMI ≥ 30 kg/m2. A greater risk of obesity, defined on the basis of %BF, occurred only in connection with longer sleep duration. The analysis of results conducted separately for both sexes revealed that in women, a greater risk of obesity was related to a longer sleep duration (≥ 9 h), whereas in men a tendency to the prevalence of obesity in connection with shorter sleep was observed (≤ 6 h). In order to test the differences in associations between sleep duration and the risk of obesity depending on sex, it is necessary to conduct further, preferably long-term studies with the use of objective methods of measurement of sleep duration.

Acknowledgments

The study was conducted with the support of the Maria Skłodowska-Curie Institute of Oncology in Warsaw and the Polish-Norwegian Foundation Research Fund (study design). The research data were collected within the scope of PONS research: “Establishing infrastructure for studies concerning health state of the population of Poland” (PNRF-228-AI-1/07) (data collection). The PONS team, who collected the data that we used in our analysis, received the funds mentioned above. The study was also supported by The Ministry of Science and Higher Education from the funds received within financing statutory activity for The Faculty of Medicine and Health Sciences, Jan Kochanowski University, research project no. 615501.00 and no. 615507.00 (analysis and preparation of the manuscript).